The invention relates to a motor vehicle having a control system for tire pressure. This control system includes the following components: at least one tire-pressure control device assigned to each tire of the motor vehicle with which the air pressure of the corresponding tire can be measured; a central control unit to which each tire-pressure control device contactlessly transmits data containing at least information as to the air pressure of the tire to which the tire-pressure control device is assigned; and, a central antenna for receiving the data transmitted by the tire-pressure control devices with this central antenna being connected to the central unit.
German patent publication 4,205,911 discloses a motor vehicle having a tire-pressure control system which includes a tire-pressure control device at each wheel of the vehicle. The tire pressure of the corresponding wheel is measured with the tire-pressure control device. Each tire-pressure control device transmits the measured value at time intervals from the wheel to the motor vehicle. The vehicle includes a receiving antenna in each wheel house to receive the transmitted data. The data, which are received by a receiver antenna, are transmitted to a central unit and are there evaluated. A warning is given to the driver of the motor vehicle when the measured air pressure at a wheel position deviates from a pregiven air pressure by a pregiven amount.
In the tire-pressure control system known from German patent publication 4,205,911, a good reception of the data transmitted by the tire-pressure control devices is ensured because a receiver antenna is disposed in the immediate vicinity of each tire-pressure control device. The tire-pressure control system known from the above publication has, however, the disadvantage that it incurs high manufacturing costs because each wheel house of the vehicle contains a receiver antenna whose acquisition and placement is expensive.
U.S. Pat. No. 5,808,190 is incorporated herein by reference and discloses a tire-pressure control system for a motor vehicle which includes the following components: at least one tire-pressure control device assigned to each tire of the motor vehicle with which the air pressure of the corresponding tire can be measured; a central control unit to which each tire-pressure control device contactlessly transmits data containing at least information as to the air pressure of the tire to which the tire-pressure control device is assigned; and, a central antenna for receiving the data transmitted by the tire-pressure control devices with this central antenna being connected to the central unit.
In the tire-pressure control system disclosed in U.S. Pat. No. 5,808,190, a pressure-control device is provided at each wheel of the motor vehicle with which the air pressure in the corresponding tire is measured. Each tire-pressure control device transmits the measured air pressure to the motor vehicle at time intervals. The motor vehicle has exactly one central antenna to receive the transmitted data. The received data are transmitted by the central antenna to a central unit and are there evaluated as explained above.
The tire-pressure control system disclosed in U.S. Pat. No. 5,808,190 brings about a low cost in the manufacture thereof because it includes only one central antenna. This antenna can be placed in the motor vehicle and connected to the central unit with little effort. The high-frequency signal transmitted by a tire-pressure control device, however, changes continuously because of the rotation of the wheel and the tire-pressure control devices have only a slight transmitting capacity. For this reason, a disturbance-free reception of the data, which are transmitted by the tire-pressure control devices, can be obtained with one central antenna only with difficulty. This publication provides no information as to how the central antenna could be configured in order to ensure a reliable reception of the data.
It is an object of the invention to provide a tire-pressure control system with tire-pressure control devices which transmit data which can be received by a central antenna of the tire-pressure control system wherein a good reception of the data by the central antenna is ensured.
The motor vehicle of the invention has a tire-pressure control system and includes: a part on the lower side of the vehicle and the part including a surface defined by an electrically insulating material; a plurality of tires; a plurality of tire-pressure control devices corresponding to respective ones of the tires for measuring the respective air pressures therein; a central unit; each one of the tire-pressure control devices being adapted to contactlessly transmit data to the central unit and the data containing at least information as to the air pressure in the tire corresponding to the one of the tire-pressure control devices; a central antenna for receiving the data transmitted by the tire-pressure control devices; and, the central antenna being connected to the central unit and being mounted on the surface.
The advantages achieved with the invention are especially that the central antenna can be well matched or tuned to the reception of the frequency or wavelength of the high-frequency signal with which the tire-pressure control devices transmit. The tuning undertaken is not affected by the base on which the central antenna is located because this base is made of an electrically-insulating material. A good reception of the data, which is transmitted by the tire-pressure control devices, is ensured because of the excellent tuning of the central antenna.
A further advantage of the invention is that a direct transmission path from the tire-pressure control device to the central antenna is ensured because of the arrangement of the central antenna beneath the vehicle at a great many angular positions of each wheel (that is, always when the wheel assumes an angular position wherein the tire-pressure control device is located outside of the wheel housing). A transmission path is provided wherein no disturbing obstructions are located for the transmitted high-frequency signal between the tire-pressure control device and the central antenna. In this way too, excellent reception of the data, which is transmitted by the tire-pressure control devices, is ensured. Still another advantage of the invention is that the tire-pressure control system can be manufactured at advantageous cost because it has only one central antenna.
According to another feature of the invention, the central antenna is mounted on a fuel tank of the motor vehicle and this fuel tank is made of electrically-insulating material. The advantage of this embodiment is that modern motor vehicles anyhow have a fuel tank made of electrically-insulating plastic whose surface is, in part, disposed below the motor vehicle so that no separate electrically-insulating surface must be made available for the central antenna. In this way, the cost of the tire-pressure control system can be still further reduced. Preferably, the central antenna is mounted on that part of the fuel tank which has the shortest spacing to the ground on which the motor vehicle stands.
According to another feature of the invention, the central antenna is impressed, utilizing an electrically-conducting material, onto an electrically-insulating foil which is attached to the electrically-insulating surface, that is, onto the fuel tank. The advantage of this embodiment is that electrically-insulating foils, onto which a central antenna is impressed, i s cost effective in manufacture. Preferably, the foil is placed on t he electrically-insulating surface or fuel tank utilizing adhesive. Preferably, the impressed central antenna is laid directly on the insulating surface or fuel tank so that it is embedded for protection against mechanical loads between the electrically-insulating surface or fuel tank and the electrically-insulating foil. In this case, an electrically-insulating adhesive is used.
According to still another feature of the invention, the central antenna is surround ed on all sides by the electrically-insulating material of the insulating surface or fuel tank. The advantage of this embodiment is that the central antenna is especially well protected against mechanical loads, for example, those loads which are caused by an impacting stone, et cetera.
In the following it will be explained how a fuel tank made of electrically-insulating material is produced with an embedded central antenna.
First, the tank is produced in a casting process, such as injection molding, with the tank being produced, for the most part, from a plastic which cannot be galvanized. Only the surface is excepted on which the central antenna is intended to lie. This surface is produced in a second process step likewise in a casting process from a plastic which can be galvanized. This casting process can, for example, be an injection molding process. Thereafter, the fuel tank is placed in a galvanizing bath so that metal deposits on the galvanizable surface and thereby the central antenna is formed on the surface of the fuel tank. Thereafter, the surface of the fuel tank on which the central antenna lies, is encased with an additional electrically-insulating plastic layer in a casting process such as an injection-molding process. The process explained here is known as a molded interconnected process and is known per se. This process is also referred to as the MID process.
According to still another feature of the invention, the central antenna is connected to the central unit via a cable, which is suitable for high frequencies, such as a coaxial cable or a twisted-pair cable. The use of a coaxial cable affords the advantage that the data, which are received by the central antenna, can be transmitted to the central unit almost loss free. The use of a twisted-pair cable affords the advantage that such a cable is especially cost effective. The twisted-pair cable is a cable which comprises two electrically-conducting individual leads which are jacketed with an electrically insulating layer and are twisted with each other.
According to still another feature of the invention, each point of the central antenna is at a distance to the neighboring electrically-conductive components of the motor vehicle (such as the chassis and the bottom sheet metal), which distance is at least one quarter of the wavelength of the high frequency signal with which the data from the tire-pressure control devices are transmitted to the central unit. The advantage of this embodiment is that the central antenna is not mismatched or detuned by neighboring electrically-conductive components of the motor vehicle. Preferably, the distance is at least half of the wavelength of the high-frequency signal with which the data are transmitted from the tire-pressure control device to the central unit.
According to another embodiment of the invention, the central antenna has the form of a dipole antenna wherein each arm is bent at least at one point. This embodiment affords the advantage that a dipole antenna configured in this manner can be accommodated on the surface of the fuel tank made of electrically-insulating material without a large requirement as to space. In this way, the above-mentioned distances to the neighboring electrically-conducting components of the chassis can be maintained without difficulties.
According to another embodiment of the invention, at least one additional central antenna is arranged on the surface of the fuel tank in addition to the first central antenna. This additional central antenna is connected to the central unit. The connection of the additional central antenna to the central unit can be made in the same way as the connection of the first central antenna to this unit (as explained above). The advantage of this embodiment is that a still greater improvement of the reception of the data, which are transmitted from the tire-pressure control devices, can be ensured with the additional central antenna.
According to another feature of the invention, each point of the additional central antenna is at a distance to each point of the first central antenna which amounts to at least one quarter of the wavelength of the high frequency signal with which the data are transmitted from the tire-pressure control devices to the central unit. Preferably, the above-mentioned distance amounts to at least half the wavelength. The advantage of this embodiment is that the first central antenna and the additional central antenna do not affect each other in a negative manner.